Fluorosilicate glass optical fibres are interesting hosts for rare-earth ions. When doped with ytterbium, for example, such fibres exhibit unique and beneficial fluoride-like spectroscopic features despite the host being predominantly silica. This work investigates the thermo-physical and chemical properties of Yb-doped fluorosilicate optical fibres in order to gain insight into the local structure about the rare earth and better understand the origins of their spectroscopic properties. Experimental findings, which suggest that the rare earth is embedded in a fluorine-rich environment in the fluorosilicate glass, are corroborated using molecular dynamic simulations. (C) 2018 Elsevier Ltd.